Improved enumeration of folder with crash reports. Safer manipulation with strings.

- Replaced `NSFileManager` `contentsOfDirectoryAtPath:error:` with `enumeratorAtURL:..` which returns ready-to-use `NSURL`s so we avoid manipulation with paths and creating of new objects.
- When composing crash report strings from new crash report files we were ignoring potential errors. Added check if actual string from file was initialized and if it is in correct format.
You are right, we don't need it. I was expecting that there will be `enumerateWithBlock:` but then didn't find it.